This invention broadly relates to turbine components other than airfoils, such as turbine disks, turbine seals and other static components, having thereon a ceramic corrosion resistant coating. This invention further broadly relates to methods for forming such coatings on the turbine component.
In an aircraft gas turbine engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against the airfoil section of the turbine blades, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward. The hotter the combustion and exhaust gases, the more efficient is the operation of the jet engine. Thus, there is incentive to raise the combustion gas temperature.
The compressors and turbines of the turbine engine can comprise turbine disks (sometimes termed “turbine rotors”) or turbine shafts, as well as a number of blades mounted to the turbine disks/shafts and extending radially outwardly therefrom into the gas flow path. Also included in the turbine engine are rotating, as well as static, seal elements that channel the airflow used for cooling certain components such as turbine blades and vanes. As the maximum operating temperature of the turbine engine increases, the turbine disks/shafts and seal elements are subjected to higher temperatures. As a result, oxidation and corrosion of the disks/shafts and seal elements have become of greater concern.
Metal salts such as alkaline sulfate, sulfites, chlorides, carbonates, oxides, and other corrodant salt deposits resulting from ingested dirt, fly ash, concrete dust, sand, sea salt, etc., are a major source of the corrosion, but other elements in the aggressive bleed gas environment (e.g., air extracted from the compressor to cool hotter components in the engine) can also accelerate the corrosion. Alkaline sulfate corrosion in the temperature range and atmospheric region of interest results in pitting of the turbine disk/shaft and seal element substrate at temperatures typically starting around 1200° F. (649° C.). This pitting corrosion has been shown to occur on critical turbine disk/shaft and seal elements. The oxidation and corrosion damage can lead to premature removal and replacement of the disks/shafts and seal elements unless the damage is reduced or repaired.
Turbine disks/shafts and seal elements for use at the highest operating temperatures are typically made of nickel-base superalloys selected for good elevated temperature toughness and fatigue resistance. These superalloys have resistance to oxidation and corrosion damage, but that resistance is not sufficient to protect them at sustained operating temperatures now being reached in gas turbine engines. Disks and other rotor components made from newer generation alloys can also contain lower levels of aluminum and/or chromium, and can therefore be more susceptible to corrosion attack.
Corrosion resistant diffusion coatings can also be formed from aluminum or chromium, or from the respective oxides (i.e., alumina or chromia). See, for example, commonly assigned U.S. Pat. No. 5,368,888 (Rigney), issued Nov. 29, 1994 (aluminide diffusion coating); and commonly assigned U.S. Pat. No. 6,283,715 (Nagaraj et al), issued Sep. 4, 2001 (chromium diffusion coating). A number of corrosion-resistant coatings have also been considered for use on turbine disk/shaft and seal elements. See, for example, U.S. Patent Application No. 2004/0013802 (Ackerman et al), published Jan. 22, 2004, which discloses metal-organic chemical vapor deposition (MOCVD) of aluminum, silicon, tantalum, titanium or chromium oxide on turbine disks and seal elements to provide a protective coating. These prior corrosion resistant coatings can have a number of disadvantages, including: (1) possibly adversely affecting the fatigue life of the turbine disks/shafts and seal elements because these prior coatings diffuse into the underlying metal substrate; (2) coefficient of thermal expansion (CTE) mismatches between the coating and the underlying metal substrate that can make the coating more prone to spalling; and (3) more complicated and expensive processes (e.g., chemical vapor deposition) for depositing the corrosion resistant coating on the metal substrate.
Accordingly, there is still a need for coatings for turbine disks, turbine shafts, turbine seal elements and other non-airfoil turbine components that: (1) provide corrosion resistance, especially at higher or elevated temperatures; (2) without affecting other mechanical properties of the underlying metal substrate or potentially causing other undesired effects such as spalling; and (3) can be formed by relatively uncomplicated and inexpensive methods.